The above-described configuration of a hybrid drive system permits a purely electrical operation using the electric machine, the disconnecting clutch being disengaged and thus the internal combustion engine not being mechanically linked to the remaining drive train of the hybrid drive system. In this state, the internal combustion engine may be turned off in order to save fuel. Since it is not mechanically linked to the remaining drive train, it transmits no (negative) drag torque, which would have to be compensated for by the electric machine.
In the event of a higher power requirement and/or when the battery associated with the electric machine is being increasingly discharged, the internal combustion engine must be started. In internal combustion engines having a starter, it is used for starting. In internal combustion engines without a starter, the start of the internal combustion engine is achieved by engaging the disconnecting clutch, the disconnecting clutch being controlled to a defined slip torque. The internal combustion engine is accelerated using this slip torque until it has reached the same rotational speed as the electric machine; the additional torque must be provided by the electric machine so that the drive torque on the hybrid drive output does not change. Since the actual slip torque of the disconnecting clutch cannot be accurately ascertained, it is not possible to activate the electric machine by simply using an appropriate additional torque. In contrast, the rotational speed of the electric machine is easily measurable and also dynamically regulatable, so that the rotational speed is regulated during the start operation.
A method is known from the related art in which the rotational speed of the electric machine is held constant at the beginning of the start operation of the internal combustion engine as a setpoint value for the entire start. The disadvantage here is that, if the motor vehicle accelerates or decelerates at the same time, the resulting slip of the torque converter changes. In the event of a simultaneous gear shift, for example, this may reach the point where the direction of slip of the torque converter changes and thus the torque is reversed, which results in a clearly perceptible jolt.
In addition, a process is known in which the development of the rotational speed is extrapolated from the rotational speed curve prior to the start operation. However, since the route traveled, such as an upward or downward slope, is not known, the result is not reliably predictable.